Alsthom's published French patent application No. 2573938 describes a limiting filter of this type. However, when using such current limiters on lines transporting high powers, it is very difficult to simultaneously satisfy requirements relating to resistance in the normal (i.e. the non-superconducting) state and requirements relating to the thermal capacity (or heat content) of the secondary winding(s). In order to keep expansion of the superconductor material and of the matrix of normal conductor metal in which it is embedded to very low values, it is the practice to ensure that the temperature of the secondary winding(s) does not rise to more than 100.degree.K in the time it takes for the circuit-breaker member to interrupt a fault current, which usually takes three or four mains cycles.
Preferred implementations of the present invention provide AC current limiters for high power transport lines in which the non-superconducting resistance of the secondary windings as seen from the primary winding is very much greater than the nominal impedance of the transformer, and in which the thermal capacity of said secondary windings is sufficiently high for their expansion to be limited to tolerable values in the event of their being heated by a short circuit current for the time it takes the circuit breaker to interrupt that current.